Wireless mobile networks, in which a user equipment (UE) such as a mobile handset communicates via wireless links to a network of base stations or other wireless access points connected to a telecommunications network, have undergone rapid development through a number of generations. The initial deployment of systems using analogue signalling has been superseded by second generation (2G) digital systems such as GSM (Global System for Mobile communications), which typically use a radio access technology known as GERA (GSM Enhanced Data rates for GSM Evolution Radio Access), combined with an improved core network.
The second generation systems have themselves been replaced by or augmented by third generation (3G) digital systems such as UMTS (Universal Mobile Telecommunications System), using the UTRA (Universal Terrestrial Radio Access) radio access technology and a similar core network to GSM. Third generation standards provide for a greater throughput of data than is provided by second generation systems, and this trend is continued with the proposals by the Third Generation Partnership Project (3GPP) of a new 4G system known as the evolved packet system (EPS) but more commonly referred to as the Long Term Evolution (LTE) system. LTE systems use an improved radio access technology known as E-UTRA (Evolved UTRA), which offers potentially greater capacity and additional features compared with the previous standards, combined with an improved core network technology referred to as the evolved packet core (EPC).
Note that the term “GERA” is used herein to refer to the radio access technology associated with GERAN (GERA networks), “UTRA” is used to refer to the radio access technology associated with UTRAN (UTRA networks), and similarly the term “E-UTRA” is used to refer to the radio access technology associated with E-UTRAN (E-UTRA networks).
As in earlier wireless mobile standards, LTE is designed as a cellular system in which base stations, known as eNBs, provide coverage over one or more cells. A mobile terminal in LTE, known as the user equipment (UE), communicates with just one base station and one cell at a time. The mobile terminal can exist in one of two communication states in LTE: an IDLE state in which the mobile terminal is basically on standby, and a CONNECTED state in which the mobile terminal is active.
In the IDLE state in LTE, the mobile terminal is tracked by the network to a specific tracking area, which may cover several base stations. The mobile terminal is not assigned to any particular base station but may itself choose which base station or base stations it listens to. The main aim in this state is to minimise signalling and resources, and thereby maximise standby time for terminals with limited battery power.
In contrast, in the CONNECTED state in LTE, the mobile terminal has a serving base station allocated to it, has its location tracked to the serving base station, and has active bearers which allow the terminal to transmit and receive at relatively high data rates. Accordingly, the terminal is actively managed by the network as it moves across a tracking area or beyond. Handover is the normal mobility mechanism in the CONNECTED state, and handles the smooth, planned transition of the terminal's connections as the terminal moves from one cell to another, and from one serving base station to another.
In LTE, the Handover mechanism is only available when the terminal is in the CONNECTED state. In the IDLE state, the terminal is free to move within the tracking area and so follows a less rigorous Cell Reselection mobility mechanism.
The Handover mechanism in LTE is managed by the network. That is, the network side determines, based on measurement data received from the user equipment, which destination cell and associated destination base station the user equipment should connect to as it moves out of range of the serving base station. From the point of view of the terminal in a Handover, the destination base station is often referred to as the target base station, while the serving base station the terminal is moving away from is referred to as the source base station.
In general, the Handover sequence involves the source base station or node initially deciding that a handover should occur, and to which target base station. The source base station then prepares the target base station for the handover by providing relevant connection information associated with the specific user equipment. Once prepared, the target base station acknowledges this to the source base station, and the source base station commands the user equipment to handover to the target base station. In response, the user equipment detaches from the source base station, and synchronises to the designated new target cell.
If a failure occurs in the mobility mechanism in the CONNECTED state, the terminal can lose its tight allocation to a serving base station. In this situation, there is a risk that the terminal may have to return to the IDLE state with an associated disruption to the data connections and potential data loss. This disruption of the connection to the source base station may be the result of a Radio Link Failure (RLF).
In order to improve the robustness of the terminal CONNECTED state, the LTE system prescribes a second mobility mechanism known as Re-establishment. Re-establishment is designed to maintain the CONNECTED state, and avoid unintended returns to the IDLE state. The Re-establishment mechanism is used to recover from several error situations in which the network failed to execute a handover in time. The Re-establishment mechanism allows a user equipment that has lost its connection with a serving base station (and has not been commanded to Handover) to reconnect to a potential target base station. A precondition for the Re-establishment to be successful is that the target base station is aware of the connection settings from the previously disrupted connection i.e. the target base station has information on the so-called UE context.
However, if there is an error in the Handover mechanism, then there is a high likelihood that the target base station will not be prepared, and will not have the relevant up-to-date connection information for the terminal to recover while in the CONNECTED state. This limits the effectiveness of the Re-establishment mechanism.
One conceivable solution to this limitation is to prepare one or more neighbouring base stations in anticipation of any potential radio link failure. However, this is difficult to achieve without creating significant additional signalling within the E-UTRAN. The number of base stations that would need to be prepared could be significant depending on the size and topography of the cells, and those base stations that are prepared are likely to need constant updating to ensure the UE context information is up-to-date.